The present invention relates generally to chemical detection systems for detecting trace amounts of chemicals, e.g., explosives or narcotics, on clothes, baggage, vehicles, shipping containers, etc. Detectors used in trace explosives detection systems include ion mobility spectrometers (IMS), mass spectrometers (MS), surface acoustic wave sensors (SAW), electron capture devices (ECD), differential mobility spectrometers (DMS), and chemiluminescence detectors (CLD).
When detecting very small (trace) quantities explosives or narcotics, the sensitivity (i.e., the amount or concentration that can be detected) and the selectivity (i.e., the correct identification of a specific chemical substance from among many other compounds in a sample) of the trace detection system are important, but often competing, factors. With a continuing need to detect even smaller and smaller amounts of explosives or narcotics, the selection of an appropriate detector becomes critical. This includes consideration of the sensitivity, selectivity, cost, size, reliability, duty cycle, and consumables. Since the chemicals of interest have become more complex, greater specificity is needed today to identify individual chemical components.
One approach for increasing detector specificity is to add additional hardware to a detector. For example, a gas chromatograph (GC) column can be added in front of an ion mobility spectrometer (IMS) detector; or an ion trap (IT) can be added in front of a mass spectrometer, to enhance specificity by delaying and, hence, spreading out the arrival times of packets of individual analytes, so that temporal overlap doesn't occur. However, this approach comes at a cost in terms of increased analysis time (typically minutes), added hardware complexity, increased space requirements, greater expenses, and increased maintenance issues.
In general, IMS detectors (and others that use similar operating principles) excel at detecting very small amounts of explosives; including, e.g., low vapor pressure explosives such as TNT, RDX, PETN, and HMX (see FIG. 1). Recently, IMS detectors have been successfully miniaturized as lightweight, hand-portable units (such as disclosed above in co-pending application Ser. No. 10/306,939, which describes a version of the Sandia National Laboratories MicroHound™ sensor platform). However, when a full-size, conventional IMS is reduced in size to a hand-portable unit, its specificity suffers. One reason is because the IMS drift tube has been shortened to about 1-2 inches (as compared to full-size, conventional IMS drift tubes that are 4-6 inches long). With standard full-length drift tubes, multiple chemicals in a complex sample physically separate into unique bunches (i.e., swarms) of individual species as they drift down the long tube. By the time the bunch hits the Faraday plate at the end of the IMS drift tube, the bunches have spread out sufficiently far so that the detector only has to identify a single chemical species. Conversely, when using a short drift tube in a miniature IMS, less physical separation occurs along the drift tube, and, hence, well-defined bunches are less likely to form. Also, with a shorter drift tube, the characteristic drift times are shorter. Consequently, the spectral peaks move closer to each other in drift-time space, and can even overlap, making identification of individual species more difficult; which also increases the rate of false alarms.
Unfortunately, the low vapor pressure explosives currently of interest (e.g., TNT, RDX, PETN, HMX) tend to have characteristic drift times that are inherently similar to one another (regardless of what length drift tube is used). This only magnifies the problem of overlapping spectral peaks when short IMS drift tubes (with short drift times) are used in miniaturized detectors. Also, background contaminants, e.g., cellulose fibers/particles from clothes and fabrics, water vapor, etc. can decompose during the detection phase and interfere with the proper identification of the target chemicals of interest (analytes). Water vapor can also attach to target analytes and affect their drift speeds. Other phenomena, such as thermal decomposition of the analyte molecules when exposed to high desorption temperatures, as well as concentration-dependent chemical reactions (e.g., dimerization of PETN) occurring inside of the IMS reaction chamber, can complicate the analysis and affect the accurate identification of individual species.
Also, there is a specific issue with IMS detectors regarding the depletion of the reactive ion population (RIP) during operation. A small amount of a dopant reactive gas, such as acetone vapor or methylene chloride vapor (depending on the chemistry) is often added to the ionization/reaction chamber of the IMS in order to improve the detector's sensitivity and overall performance, by enhancing the creation of negatively-charged analyte ions (when the detector is operated in the negative-ion mode). As the reactive ions charge-exchange with the analyte molecules, the population of reactive ions is depleted, and the population of analyte ions increases. However, if the reactive ion population drops too low, then the sensitivity of the detector drops dramatically and remains there until the reactive ion population recovers sufficiently. This situation (i.e., excessive depletion) can occur, e.g., when the ionization/reaction chamber is overloaded by an excessively large number of incoming analyte gas molecules (or, for that matter, when overloaded with other background gases or other gases not of interest).
The problem of excessive reactive ion depletion can be addressed, for example, by not presenting the detector with an excessively high concentration of analyte molecules. However, IMS detectors are concentration-dependent devices, meaning that the greater the concentration of incoming analyte gas, the greater the signal to noise (S/N) ratio is. So, these two conflicting requirements (i.e., low analyte concentration to keep reactive ion population high versus high analyte concentration to get a high signal) require careful optimization of the system's design and performance characteristics.
Two different methods are commonly used to collect samples of unknown chemicals, depending on if they are particles or vapors. Small particles are typically collected by swiping a small piece of cotton cloth or flexible metallic mesh across a contaminated surface. Vapors (as well as particles) are typically collected and pre-concentrated by flowing (i.e., moving, vacuuming) contaminated air through a porous metallic filter mesh (such as a stainless steel mesh, felt, or screen). Low vapor pressure explosive molecules are “sticky”, meaning that they easily adsorb onto the wires of a metallic mesh. On the other hand, high vapor pressure explosives (see FIG. 1) typically pass through the preconcentrator screen without sticking, which makes them more difficult to collect and pre-concentrate. During collection, concentrated puffs of air can be directed towards a surface to dislodge particles lying on the surface or stuck in clothing, fabrics, etc., which can then be sucked into the preconcentrator module.
Next, in some devices, the contaminated mesh is removed from the preconcentrator module and then placed in a thermal desorption chamber located close to (or, as part of) the detector. Alternatively, the mesh can be heated inside of a combined collection/preconcentrator module without removing the mesh. In either case, we define “desorption chamber” as the location where the contaminated mesh is heated to thermally desorb the collected contaminants. In the desorption chamber, the metallic mesh is heated to about 180 C to 220 C to vaporize and desorb the contaminants. Depending on how fast the mesh is heated up, the contaminants may be released quickly or slowly. Conventionally, the mesh is rapidly heated (i.e., flash heated) in a single short pulse from room temperature to about 200-210 C over a very short period of time, e.g., 0.2-0.4 seconds). When flash heated, almost all of the collected particles and adsorbed vapors are released at essentially the same time; thereby generating a single, concentrated pulse (i.e., packet, bunch, or group) of analyte gas molecules. While the preconcentrator mesh is being heated, a carrier gas (e.g., clean, dry air, nitrogen, helium, etc.), flows through, or across, the mesh and carries the desorbed contaminants along a short gas transfer tube to the chemical detector (such as a ion mobility spectrometer (IMS) or mass spectrometer (MS)).
The metallic preconcentrator mesh is typically heated by flowing a high-amperage electric current through the stainless-steel mesh wires to generate internal heat by Joule-type electric resistance heating. For example, a 12-volt gel-cell type battery can be used to provide 60-80 amps of current through a stainless steel mesh; which is sufficient to raise the peak mesh temperature to about 200 C in about 0.2-0.4 seconds. Alternatively, the mesh may be heated to about 200 C even more rapidly, e.g., in less than 0.01 seconds with concentrated light from a laser or flash lamp (e.g., Xenon lamp).
As explained above, the flash heating of the preconcentrator mesh generates a much greater concentration of analyte gas than could be collected by continuous air sampling and simultaneous detection. Hence, by flash desorbing a metallic preconcentrator mesh, the signal-to-noise (S/N) ratio of the detector can be increased by a factor of 1000× or more; as compared to continuously sampled systems that don't use a preconcentrator mesh. However, when the preconcentrator mesh is flash heated, essentially all of the different species of unknown explosive compounds are released at the same time. While this results in a high concentration of analyte gas presented to the detector, the near-simultaneous arrival of the contaminants can cause the resulting spectral peaks of the IMS spectrum to bunch up and overlap. This makes it more difficult to separate out and identify individual chemical species (such RDX and PETN, which have similar characteristic ion mobility drift times). Also, flash desorption may release too much gas all at once and wipe out the population of dopant reactive ions in the IMS. If a second batch of analyte molecules were to be subsequently sent in while the reactive ion population was depleted, the subsequent signal generated could be too small for an IMS to detect and analyze.
Another consideration is to minimize the detection system's duty cycle (i.e., the turnaround time required to collect and analyze a sample), in order to more rapidly process large numbers of people, baggage, cars, etc. This becomes especially important for detectors used airports, border crossings, etc., which require high throughput and low false alarm rates. Hence, the detector analysis time, including the thermal desorption step, should be as short as possible.
Also, when the mesh is flash heated to 200 C in 0.2-0.4 seconds, some undesirable chemistry can happen that may result in a more complicated mobility spectrum due to the presence of additional peaks. This can affect the ability of the IMS detection system to identify individual species. For some explosives (e.g., PETN), decomposition occurs at the higher temperatures (especially when approaching 200 C). These decomposition products create extra spectral peaks that would be more prominent when flash heating the mesh, as compared to slowly heating the mesh over a much longer period of time, e.g., 10 seconds, because the decomposition products wouldn't show until much later.
Additionally, there are some concentration related chemical reactions (e.g., dimerization of PETN) that can occur in the drift region of the IMS, which occur more readily when the mesh is flash heated. When flash heated, all of the PETN is volatilized essentially at once, releasing a single packet at a higher concentration to IMS; hence, the probability of PETN molecule-to-molecule collisions is greater at higher concentrations. Alternatively, when a much slower (e.g., 10 s), stepped-temperature profile is used, each individual packet of PETN molecules being sent into the IMS each time, when the temperature is stepped up, has a lower concentration, and, hence, a lower chance of dimerization due to PETN molecule-to-molecule collisions.
What is needed, then is a way to reduce the problem of too closely-spaced real-time and drift time peaks caused by flash desorption of a preconcentrator mesh (i.e., releasing all of the species at once); without increasing the length of the IMS drift tube; without adding too much additional hardware or cost; and without increasing the duty cycle time too much; while, at the same time, allowing the reactive ion population to recover sufficiently in-between heating pulses; reducing decomposition of target molecules at high mesh temperatures; and reducing dimerization of target molecules at high concentrations inside of the IMS; both of which can create additional spectral peaks that can confuse the analysis and identification of individual target species.